Against the background of global environmental problems, reduction of exhaust gas is required for automobiles. Technologies on a diagnosing function monitoring exhausting performance in a practical use environment on a real-time basis and notifying a driver of deterioration of the exhaust performance to a constant level or above have been developed until now.
By highly efficiently utilizing a catalyst furnished in an exhaust pipe, nearly 100% of exhaust gas components of HC, CO, NOx can be purified. Because a catalyst is activated and exerts purification performance when its temperature becomes 200-300° C. or above, exhaust performance from the time of engine start to activation of the catalyst predominantly determines exhaust performance of the engine. Accordingly, monitoring of the exhaust performance at the time of engine start on a real-time basis becomes important. At the time of engine start, HC performance is particularly important. In JP-A No. 2007-170363, a means is disclosed which diagnoses presence/absence of abnormality of a rapid catalyst warm-up control means based on a ratio of engine load and engine speed during idling operation.